1. Field of the Invention
The present invention relates to a diffractive optical element having improved diffraction efficiency in a wide wavelength range and an image pickup optical system using the diffractive optical element.
2. Description of the Related Art
Previously, as a method of reducing a chromatic aberration of an optical system, a method of providing a diffractive optical element that has a diffraction function as apart of the optical system has been known (SPIE Vol. 1354, International Lens Design Conference (1990)). The diffractive optical element is manufactured by making a shape of a diffraction grating on a mold by cutting or the like and then transferring the shape into a resin by a molding process such as an UV curing, a thermal curing, or an injection molding. When the diffractive optical element is manufactured by this molding process, a refractive index distribution is generated inside a diffraction grating. Particularly, when the molding is performed by the UV curing using a material in which inorganic nanoparticles are dispersed into a base resin material of the diffraction grating, the nanoparticles move during the curing process and therefore the concentration of the nanoparticles is different in accordance with an area of the diffraction grating. Since the nanoparticles have a refractive index that is different from a refractive index of the base resin material, eventually the refractive index distribution is generated.
Japanese Patent Laid-Open No. H11-48355 discloses a diffractive optical element that has a corrected shape. The shape is manufactured by previously deforming a mold in an opposite direction with respect to a shift from a design shape that is generated when a protection film is formed to obtain a design shape after molding the diffractive optical element. Japanese Patent Laid-Open No. 2008-180963 discloses a diffractive optical element that is formed by previously correcting a shape of a mold in an opposite direction in accordance with a change of a film thickness of an anti-reflection film in order to suppress the deterioration of diffraction efficiency caused by non-uniformity of the film thickness of the anti-reflection film. Japanese Patent No. 3252708 discloses a scanning optical system that corrects an image plane movement caused by an internal strain that is generated during molding an optical element. In the scanning optical system disclosed in Japanese Patent No. 3252708, the refractive index distribution that is generated when the optical element is formed by using an injection mold is corrected by previously shifting a focal length.
However, in any of Japanese Patent Laid-Open No. H11-48355, Japanese Patent Laid-Open No. 2008-180963, and Japanese Patent No. 3252708, when the refractive index distribution exists in the diffraction grating, an optical path length of transmitted light that transmits through the diffraction grating is shifted from a design value and therefore the diffraction efficiency is deteriorated.